Hello, Chris. Welcome.
That would be a West Virginia swirl. Likely from 1930's to 1950's.
They're called West Virginia swirls because there were many companies located in WV making somewhat similar companies. About 14 companies if I recall correctly.
There's a good chance that the company which made your marble is Alley Agate.
My second choice would be Ravenswood.
To be safe though, I'll stick with WV Swirl.